This invention relates to reactive multilayer foils, especially freestanding multilayer foils, useful as local heat sources.
Reactive multilayer coatings are useful in a wide variety of applications requiring the generation of intense, controlled amounts of heat in a planar region. Such structures conventionally comprise a succession of substrate-supported coatings that, upon appropriate excitation, undergo an exothermic chemical reaction that spreads across the area covered by the layers generating precisely controlled amounts of heat. While we will describe these reactive coatings primarily as sources of heat for welding, soldering or brazing, they can also be used in other applications requiring controlled local generation of heat such as propulsion and ignition.
In many methods of bonding or joining materials, a heat source is required. This heat source may either be external or internal to the structure to be joined. When external, the heat may be generated from a device such as a furnace. Processes incorporating such heat sources require the heating of the entire unit to be bonded, including the bulk materials and the bond material, to a temperature high enough to melt the bond material. Such a method presents problems because the bulk materials to be joined are often delicate or sensitive to the high temperatures required in the process. These high temperatures may damage the materials to be bonded.
To alleviate the problems associated with external heat sources, internal heat sources are utilized. These heat sources often take the form of reactive powders or even electrical wires. When reactive powders are used, a mixture of metals or compounds that will react exothermically in a self-propagating reaction to form a final compound or alloy is utilized. Such processes have existed since self-propagating powders were developed in the early 1960s, spawning what is known as Self-Propagating, High-Temperature Synthesis (SHS). SHS reactions, however, often require substantial preheating to self-propagate, and controlling the rate and manner in which their energy is released is often difficult. As a result, bonding may be inconsistent or insufficient.
To combat the problems associated with reactive powder bonding, multilayer structures comprised of materials, which allow similar exothermic reactions, have been developed. Such structures allow for more controllable and consistent heat generating reactions. The basic driving force behind such SHS reactions is a reduction in atomic bond energy. When a structure having a series of layers of reactive material (known as a foil) is ignited, heat is produced as the distinct layers atomically combine. This heat ignites adjacent regions of the foil, thereby allowing the reaction to travel the entire length of the structure, generating heat until all material is reacted. Even with such advances in bonding technology, however, there remain problems. Many materials, for example, posed major difficulties and previously could not be successfully bonded. Additionally, methods utilizing reactive foils as heat sources often resulted in the foil debonding from the substrate upon reaction, thereby weakening the bond. Accordingly there is a need for improved reactive multilayer foils.
Reactive foils and their uses are provided as localized heat sources useful, for example, in ignition, joining and propulsion. An improved reactive foil is preferably a freestanding multilayered foil structure made up of alternating layers selected from materials that will react with one another in an exothermic and self-propagating reaction. Upon reacting, this foil supplies highly localized heat energy that may be applied, for example, to joining layers, or directly to bulk materials that are to be joined. This foil heat-source allows rapid bonding to occur at room temperature in virtually any environment (e.g., air, vacuum, water, etc.). If a joining material is used, the foil reaction will supply enough heat to melt the joining materials, which upon cooling will form a strong bond, joining two or more bulk materials. If no joining material is used, the foil reaction supplies heat directly to at least two bulk materials, melting a portion of each bulk, which upon cooling, form a strong bond. Additionally, the foil may be designed with openings that allow extrusion of the joining (or bulk) material through the foil to enhance bonding.